After merging #410 we have Roslyn 3.0.0 and C# 8 so we are able to handle nullable references.
The problem is, even if you enable it with #nullable in a CSX file, it emits warnings and at the moment we never show any warnings.

Possible options:

• we start always showing warnings too (and potentially add an opt out flag to stop showing them). I like this, but it's potentially a breaking change (different behavior)
• we add an opt in flag to show warnings
• we add an opt in flag to treat warnings as errors
• we add an opt in flag for "nullable references" and we implicitly show warnings + treat warnings as errors then. The advantage of that would be that the code wouldn't need to have the #nullable pragma then.